Near-optimal low-complexity decoding of space-time codes for fixed wireless applications

ABSTRACT

An improved multi-antenna receiver is realized for detecting signals transmitted by a multi-antenna transmitter by summing signals received at the plurality of receiver antennas after multiplying each by a respective constant. The summed signal is applied to a maximum likelihood detector. The respective constants, λ j , where j is an index designating a particular receiver antenna, are determined by evaluating the largest eigenvector of the matrix A, where Λ is a vector containing the values λ j , and A is a matrix containing elements α ij , which is the transfer function between the i th  transmitter antenna to the j th  receiver antenna. The α ij  terms are determined in the receiver in conventional ways.

REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 60/068,613, filed Dec. 23, 1997.

BACKGROUND OF THE INVENTION

[0002] This invention relates to wireless systems and, more particularly, to systems having more than one antenna at the receiver and at the transmitter.

[0003] Physical constraints as well as narrow bandwidth, co-channel interference, adjacent channel interference, propagation loss and multi-path fading limit the capacity of cellular systems. These are severe impairments, which liken the wireless channel to a narrow pipe that impedes the flow of data. Nevertheless, interest in providing high speed wireless data services is rapidly increasing. Current cellular standards such as IS-136 can only provide data rates up to 9.6 kbps, using 30 kHz narrowband channels. In order to provide wideband services, such as multimedia, video conferencing, simultaneous voice and data, etc., it is desirable to have data rates in the range of 64-144 kbps.

[0004] Transmission schemes for multiple antenna systems may be part of a solution to the problem of the currently available low data rates. Such schemes were first proposed in papers by Wittneben, and by Seshadri and Winters, where the problem was addressed in the context of signal processing.

[0005] One prior art arrangement having a single transmitter antenna and multiple receiver antennas is shown in FIG. 1. Each of the receiver antennas receives the transmitted signal via a slightly different channel, where each channel i is characterized by transfer function α_(i). Using an approach known as “Maximum Ratio Combining”, the prior art approach to detection contemplates multiplying each received signal that had been influenced by α_(i) by the complex conjugate signal, α_(i)*, summed, and then processed.

[0006] In a co-pending application titled “Method and Apparatus for Data Transmission Using Space-Time Codes and Multiple Transmit Antennas”, filed on May 6, 1997, bearing the Ser. No. 08/847,635, and assigned to the assignee of this invention, a coding perspective was adopted to propose space-time coding using multiple transmit and receive antennas. Space-time coding integrates channel coding, modulation, and multiple transmit antennas to achieve higher data rates, while simultaneously providing diversity that combats fading. It may be demonstrated that adding channel coding provides significant gains over the schemes of Wittneben and Seshadri and Winters. In said co-pending application, space-time codes were designed for transmission using 2-4 transmit antennas. These codes perform extremely well in slowly varying fading environments (such as indoor transmission media). The codes have user bandwidth efficiencies of up to 4 bits/sec/Hz which are about 3-4 times the efficiency of current systems. Indeed, it can be shown that the designed codes are optimal in terms of the trade-off between diversity advantage, transmission rate, decoding complexity and constellation size.

[0007] It can also be shown that as the number of antennas is increased, the gain increases in a manner that is not unlike a multi-element antenna that is tuned to, say, a particular direction Unfortunately, however, when maximum likelihood detection is employed at the receiver, the decoding complexity increases when the number of transmit and receive antennas is increased. It would be obviously advantageous to allow a slightly sub-optimal detection approach that substantially reduces the receiver's computation burden.

SUMMARY

[0008] Such an approach is achieved with a receiver arrangement where signals received at a plurality of antennas are each multiplied by a respective constant and then summed prior to being applied to a maximum likelihood detector. The respective constants, λ_(j), where j is an index designating a particular receiver antenna, are derived from a processor that determines the largest eigenvalue of the matrix ΛA(Λ*)^(T), where A is a vector containing the values λ_(j), and A is a matrix containing elements α_(ij), which is the transfer function between the i^(th) transmitter antenna to the j^(th) receiver antenna. The α_(ij) terms are determined in the receiver in conventional ways.

BRIEF DESCRIPTION OF THE DRAWING

[0009]FIG. 1 presents a block diagram of Maximal Ratio Combining detection; and

[0010]FIG. 2 presents a block diagram of an arrangement including a transmitter having a plurality of antennas, and a receiver having a plurality of antennas coupled to an efficient detection structure.

DETAILED DESCRIPTION

[0011]FIG. 1 presents a block diagram of a receiver in accord with the principles of this invention. It includes a transmitter 10 that has an n plurality of transmitting antenna 1, 2, 3, 4, and a receiver 20 that has an m plurality of receiver antennas 21, 22, 23, 24. The signals received by the receiver's antennas are multiplied in elements 25, 26, 27, and 28, and summed in adder 30. More specifically, the received signal of antenna j is multiplied by a value, λ_(j), and summed. The collection of factors λ_(j) can be viewed as a vector Λ. The outputs of the receiver antennas are also applied to processor 40 which, employing conventional techniques, determines the transfer functions α_(ij) for i=1, 2, 3, . . . , n and j=1, 2, 3, . . . , m. These transfer functions can be evaluated, for example, through the use of training sequences that are sent by the different transmitter antennas, one antenna at a time.

[0012] The evaluated α_(ij) signals of processor 40 are applied to processor 45 in FIG. 1 where the multiplier signals λ_(j), j=1, 2, 3, . . . , m are computed. Processor 45 also evaluates a set of combined transfer function values γ_(i), i=1, 2, 3, . . . , n (which are described in more detail below). Signals γ_(i) of processor 45 and the output signal of adder 30 are applied to detector 50 which detects the transmitted symbols in accordance with calculations disclosed below.

[0013] It is assumed that the symbols transmitted by the antennas of transmitter 10 have been encoded in blocks of L time frames, and that fading is constant within a frame. A codeword comprises all of the symbols transmitted within a frame, and it corresponds, therefore, to

c₁ ¹c₁ ²c₁ ³ . . . c₁ ⁴c₂ ¹c₂ ²c₂ ³ . . . c₂ ⁴c₃ ¹c₃ ²c₃ ⁴ . . . c₃ ⁴ . . . c_(m) ¹c_(m) ²c_(m) ³ . . . c_(m) ⁴,  (1)

[0014] where the superscript designates the transmitter's antennas and the subscript designates the time of transmission (or position within a frame).

[0015] From the standpoint of a single antenna, e.g., antenna 1, the signal that is received at antenna 1 in response to a transmitted symbol c_(t) ¹ at time interval t is: $\begin{matrix} \begin{matrix} {R_{t} = {c_{t}^{1}\left( {{\alpha_{11}\lambda_{1}} + {\alpha_{12}\lambda_{2}} + {\alpha_{13}\lambda_{3}} + \ldots + {\alpha_{1m}\lambda_{m}}} \right)}} \\ {= {c_{t}^{1}{\sum\limits_{j = 1}^{m}{\lambda_{j}\alpha_{1j}}}}} \\ {= {c_{t}^{1}\gamma_{1}}} \end{matrix} & (2) \end{matrix}$

[0016] (when noise is ignored). If each λ_(j) value is set to α*_(1j) (where α*_(1j) is the complex conjugate of α_(1j)) then the received signal would simply be $\begin{matrix} {R_{t} = {c_{t}^{11}{\sum\limits_{i = 1}^{m}{\alpha_{11}}^{2}}}} & (3) \end{matrix}$

[0017] yielding a constructive addition.

[0018] Of course, the values of λ₁ cannot be set to match α*_(1j) and concurrently to match the values of α*_(1j) where i≠1; and therein lies the difficulty.

[0019] When all n of the transmitting antennas are considered, then the received signal is $\begin{matrix} \begin{matrix} {R_{t} = {\sum\limits_{i = 1}^{n}\left( {c_{t}^{i}{\sum\limits_{j = 1}^{m}{\lambda_{j}\alpha_{ij}}}} \right)}} \\ {= {\sum\limits_{i = 1}^{n}{c_{t}^{i}\gamma_{i}}}} \end{matrix} & (4) \end{matrix}$

[0020] In accordance with the present disclosure, the objective is to maximize $\sum\limits_{i = 1}^{n}{\gamma_{i}}^{2}$

[0021] because by doing so, signal R_(i) contains as much information about c₁ ^(i),=1,2,3, . . . n as is possible. However, it can be easily shown that if a matrix A is constructed such that $\begin{matrix} {{A = {\sum\limits_{i = 1}^{n}{\left( \Omega_{i}^{*} \right)^{T}\Omega_{i}}}},} & (5) \end{matrix}$

[0022] where Ω_(i)=(α_(i1),α_(i2), α_(i3) . . . α_(im)), then $\begin{matrix} {{\sum\limits_{i = 1}^{n}{\gamma_{i}}^{2}} = {\Lambda \quad {{A\left( \Lambda^{*} \right)}^{T}.}}} & (6) \end{matrix}$

[0023] The receiver, thus, has to maximize ΛA(Λ*)^(T), subject to the constraint ∥Λ∥²=1. The solution to this problem is to choose Λ to be the eigenvector of A which corresponds to the maximum eigenvalue of A. Accordingly, processor 45 develops the matrix A from the values of α_(ij), finds the eigenvalues of A in a conventional manner, selects the maximum eigenvalue of A, and creates the vector A. Once Λ is known, processor 45 develops signals γ_(i) for 1=1, 2, 3, . . . , n, (where $\gamma_{i} = {\sum\limits_{j = 1}^{m}{\lambda_{j}\alpha_{ij}}}$

[0024] ), and applies them to detector 50. Finally, detector 50 minimizes the metric $\sum\limits_{t = 1}^{L}{{R_{t} - {\sum\limits_{i = 1}^{n}{\gamma_{i}c_{t}^{i}}}}}^{2}$

[0025] from amongst all possible codewords in a conventional manner. As can be seen, this approach reduces the complexity of decoding by almost a factor of m.

[0026]FIG. 1 depicts separate multipliers to multiply received signals by multiplication factors λ_(i), and it depicts separate blocks for elements 30, 40, 45, and 50. It should be understood, however, that different embodiments are also possible. For example, it is quite conventional to incorporate all of the above-mentioned elements in a single special purpose processor, or in a single stored program controlled processor (or a small number of processors). Other modifications and improvements may also be incorporated, without departing from the spirit and scope of the invention, which is defined in the following claims. 

1-6. (Canceled)
 7. A method of detecting symbols transmitted wirelessly from n number of transmitting antennas, the method comprising: receiving transmitted signals from the n number of transmitting antennas by an m number of receiving antennas; determining transfer functions α_(ij) for each wireless data channel by processing the received signals, wherein each wireless data channel is a path from one transmitting antenna to one receiving antenna; forming an n×m matrix A, wherein the channel transfer functions α_(ij) represent elements of the matrix A; finding an eigenvector Λ associated with a maximum eigenvalue of matrix A, wherein λ_(j) represent m elements of the eigenvector Λ; computing S, wherein S is an inner product of the eigenvector Λ and a vector whose element ξ_(j) is the signal received by the j^(th) receiving antenna; computing γ_(i)'s, wherein γ_(i) is an inner product of a i^(th) row of the A matrix and the eigenvector Λ; and detecting the transmitted symbols, utilizing γ_(i)'s and S under a maximum likelihood detection scheme.
 8. The method of claim 7, wherein the transmitting antennas transmit encoded symbols in blocks of multiple time frames, and wherein a codeword comprises all encoded symbols transmitted within a time frame.
 9. The method of claim 7, wherein the wireless signal transmitted by the transmitting antennas is encoded under a space-time modulation scheme.
 10. A system of processing wireless transmitted data, the system comprising: m number of transmitting antenna; n number of receiving antenna; at least one channel estimator; at least one processor; a maximum likelihood detector; wherein, under the system: each receiving antenna receives signals from the m transmitting antennas; the at least one channel estimator, utilizing the received signals, determines a transfer function of each data path from each transmitting antenna to each receiving antenna; the at least one processor determines an eigenvector Λ associated with a maximum eigenvalue of a matrix A, wherein the transfer functions represent elements of the matrix A; the at least one processor computes a product of the eigenvector Λ and a vector whose elements are the signals received by the receiving antennas; the at least one processor computes inner products of each row of the A matrix and the eigenvector Λ; and the maximum likelihood detector detects transmitted symbols in from the received signals utilizing the computed inner products.
 11. A signal processing apparatus for use in a wireless receiver, wherein the wireless receiver forms part of a wireless system having a wireless transmitter employing multiple transmitting antennas, and wherein the wireless receiver includes two or more receiving antennas for receiving signals transmitted from the multiple transmitting antennas, the signal processing apparatus comprising: an input section for receiving multiple signals provided by the two or more receiving antennas; a processing section that is coupled to the input section to develop multiplying values from transfer function values associated with the multiple transmitting antennas and associated with the two or more receiving antennas, wherein the processing section develops the multiplying values based on a less than optimal computational process; a multiplying section, coupled to the input section and to the processing section, that multiplies the received signals by the multiplying values to produce multiplied received signals; and a summing section, coupled to the multiplying section, that receives and adds the multiplied received signals.
 12. The apparatus of claim 11, further comprising: developing a matrix from the transfer function values; finding eigenvalues of the matrix; creating a maximum eigenvector of the matrix; and generating a subset of the set of all possible values of the received encoded symbols from the maximum eigenvector.
 13. The apparatus of claim 11, wherein the multiple transmitting antennas transmit encoded symbols in blocks of multiple time frames, and wherein a codeword comprises all encoded symbols transmitted within a time frame.
 14. The apparatus of claim 11, wherein the transfer function values comprise: a matrix of eigenvectors associated with the transfer function values.
 15. A system for processing wireless data, the system comprising: means for receiving at an m number of receiving antennas a wireless signal, wherein the wireless signal represents multiple codewords; and means, coupled to the means of receiving, for processing the wireless signal to determine the codewords, under a less than optimal computational process, wherein a number of computations is reduced by approximately a factor of m, at an increase in less than a factor of m in frame error probability from the optimal computational process, and wherein the optimal computational process computes all codewords.
 16. The system of claim 15, wherein the wireless signal is transmitted by multiple transmitting antennas, and the wireless signal is encoded under a space-time modulation scheme.
 17. The system of claim 15, wherein the means for processing includes means for computing eigenvectors based on the m number of receiving antennas.
 18. The system of claim 15, wherein the wireless signal is transmitted by multiple transmitting antennas that transmit encoded symbols in blocks of multiple time frames, and wherein a codeword comprises all encoded symbols transmitted within a time frame.
 19. An apparatus for receiving encoded symbols from multiple transmitting antennas under a wireless communication system, the apparatus comprising: one or more electronic circuits, wherein the electronic circuits include: an input portion to receive input signals from each one of multiple receiving antennas, wherein a transfer function is associated with each transmitting antenna-receiving antenna pair; a first signal processing section for generating multiple transfer function values representing channels over which the input signals are received, wherein each transfer function is associated with a transmitting-receiving antenna pair associated with the received encoded symbols; a second signal processing section for generating multiple combined transfer function values generated from combining the transfer functions such that a number of decoding computations is reduced; a multiplier for multiplying the received input signals with a respective combined transfer function value; an adder coupled to the multiplier for adding the multiplied signals; and an output portion for outputting the added signals for decoding.
 20. The apparatus of claim 19, wherein the second signal processing section generates multiple combined transfer function values by developing a matrix from the transfer function values, finding eigenvalues of the matrix, creating a maximum eigenvector of the matrix, and generating a subset of values from a set of all possible values of received encoded symbols from the maximum eigenvector.
 21. The apparatus of claim 19, wherein the input signals include transmitted codewords, wherein the multiple transmitting antennas transmit the encoded symbols in blocks of multiple time frames, and wherein a codeword comprises all encoded symbols transmitted within a time frame.
 22. The apparatus of claim 19, wherein the input signals include transmitted codewords, wherein the multiple transmitting antennas transmit the encoded symbols in blocks of multiple time frames, and, wherein a codeword comprises c₁ ¹ c₁ ² c₁ ³ . . . c₁ ⁴ c₂ ¹ c₂ ² c₂ ³ . . . c₂ ⁴ c₃ ¹ c₃ ² c₃ ³ . . . c₃ ⁴ . . . c_(m) ¹ c_(m) ² c_(m) ³ . . . c_(m) ⁴.
 23. The apparatus of claim 19, further comprising a decoding circuit coupled to the output portion, wherein the decoding circuit is configured to compute a subset from a set of all possible values of codewords associated with the input signals and by employing the added signals and the combined transfer function values.
 24. The apparatus of claim 19, further comprising a decoding circuit coupled to the output portion and configured to compute a subset from a set of all possible values of codewords associated with the input signals, wherein the combined transfer function values are designated ${\gamma_{i} = {\sum\limits_{j = 1}^{m}{\lambda_{j}\alpha_{i\quad j}}}},$

and wherein the decoding circuit computes a subset from a set of all possible values by minimizing $\sum\limits_{t = 1}^{L}{{R_{t} - {\sum\limits_{i = 1}^{n}{\gamma_{i}c_{t}^{i}}}}}^{2}$

from among all possible codewords.
 25. The apparatus of claim 19, further comprising a maximum likelihood detector coupled to the output portion.
 26. A method of processing wirelessly transmitted data, wherein the data is transmitted from multiple transmitting antennas and is received by multiple receiving antennas, the method comprising: receiving transmitted signals from the transmitting antennas by the receiving antennas; determining channel behaviors, utilizing the received signals, wherein each channel behavior represents an effect of one particular transmission path, from one of the transmitting antennas to one of the receiving antennas, on the received signal; computing a weight factor for each receiving antenna based on a combination of all channel behaviors; computing receiver factors based on the received signals and transmission paths from the transmitting antennas to the receiving antennas; computing a transmitter factor related to each transmitting antenna based on the computed receiver factors and the channel behaviors of paths between the particular transmitting antenna and all the receiving antennas; and detecting transmitted symbols from the received signals, utilizing the transmitter factors, the weight factors, and a summation of the received signals, and based on statistics or probability properties.
 27. The method of claim 26, wherein the processing includes computing eigenvectors based on the m number of receiving antennas.
 28. The method of claim 26, wherein the wireless signal is transmitted by multiple transmitting antennas, and the wireless signal is encoded under a space-time modulation scheme. 